Many conventional slimes and sludges are incapable of being dewatered by subjecting them to ambient evaporation. For instance, phosphate slime produced during the production of fertilizer or the like from phosphate rock, will not dry by ambient evaporation even over periods as long as 40 years, and when land filled even after such a long period of exposure to ambient air does not allow the construction of homes or the utilization of heavy equipment on the landfilled area. The fluid boundary layer on the solids in the slimes and sludges is the phenomenon responsible for such water-retaining characteristics of many conventional slimes and sludges, and additionally the boundary layer fluid absorbs relatively high amounts of dissolved matter such as heavy metal salts and dissolved organics. While some slimes and sludges can be subjected to centrifugal force, high pressure, or heat to remove the liquid and/or heavy metal salts and dissolved organics held in place by the solids in the sludge or slime, a very large amount of energy must be applied in order to effect such removal, and in some circumstances (e.g., with phosphate slime) feasible successful methods do not exist for effecting such removal.
Since the holding or bonding forces retaining the liquid with dissolved organics and heavy metal salts in the sludge or slime are of an electrostatic nature, induction, electrostriction, and electrophoresis have been tried in the past to effect dewatering; for instance, attempts at electrophoretic sludge treatment have been made as far back as 1940. However, such prior attempts have been incapable of practically effecting sludge or slime dewatering, in part due to the lack of a suitable anode material (coated valve-metal anodes are now available which overcome that deficiency, however), and because neutral zones of water were formed within the cathode region, accumulation of the water interrupting the continuity of the electrophoretic transport which is dependent upon continuous boundary layers.
According to the present invention, it is possible to effectively dewater a wide variety of slimes and sludges, such as phosphate slime, sewage sludge, sludge from water treatment plants, sludge from paper mills, aluminum hydroxide containing clay sludge, dredging slimes (oily slimes) and the like, and it is possible to effect such dewatering with a very small amount of applied energy (e.g., 200 kwh or less per 1000 gal.). Also, in effecting dewatering the heavy metals contained in the boundary layer liquid also are removed with the liquid, and depending upon the original nature of the sludge, the sludge may be suitable for fertilizer or other end uses, and the heavy metals or other valuable materials (e.g., phosphoric acid) may be removed from the withdrawn liquid.
According to the present invention, dewatering apparatus for sludge or the like is provided which comprises first and second electrodes; means connecting the electrodes to a source of e.m.f.; means for maintaining the sludge or the like to be treated between the electrodes during treatment; means for providing a continuation of the boundary layer of the sludge disposed between the electrodes, said means allowing the passage of water therethrough, and being disposed in intimate contact with at least one of the electrodes between the electrode and the sludge; and means for ensuring continuous contact between the electrodes and the sludge during treatment even though the volume of the sludge decreases during treatment due to water removal. For most effective removal, the e.m.f. source is a DC source, with the boundary layer continuation means disposed at least in intimate contact with the cathode, and the cathode being constructed to allow passage of water therethrough and being located vertically below the anode. The boundary layer continuation means comprises a layer of material that is porous, fibrous, or strongly absorbent. For porous materials a pore size of about 0.01 to 5 mm is preferred. Suitable exemplary boundary layer continuation means include asbestos cloth, sand, filter paper, and a wide variety of webs of woven or nonwoven inert material including cotton, wool, and polyester webs.
Further, means for effecting continuous feeding and discharge of sludge to and from the means for maintaining the sludge between the electrodes can be provided, such means comprising a pair of conveyor belts operatively disposed between the electrodes for continuously moving sludge to be treated between the electrodes, and means for mounting the conveyor belts and electrodes so that the volume between the conveyor belts decreases from an inlet thereto to an outlet therefrom. The actual conveyor belt itself associated with the cathode preferably provides the boundary layer continuation means. The means for ensuring continuous contact between the electrodes and the sludge preferably comprises a pneumatic cylinder biasing (as opposed to providing large compressive forces) at least a portion of the electrodes together.
According to a method of dewatering slime or sludge according to the present invention, the slime or sludge is settled, and the water produced by the settling operation removed; the slime or sludge is fed between a pair of electrodes while applying a current to the electrodes, at least one of the electrodes having means providing for a continuation of the boundary layer of the slime or sludge between the electrodes; the contact between the electrodes and the sludge is maintained even during volume decreasing of the sludge do to water removal; and the water from the sludge during dewatering is removed through at least one of the electrodes. Where the sludge or slime being treated is phosphate slime, the method comprises the further step of subjecting the slime, after dewatering, to electrodialysis to produce phosphoric acid, the water removed during the settling and dewatering steps being reused for the ultimate further production of phosphate slime (during the production of fertilizer or the like from phosphate rock). Where the sludge or slime being treated is sludge containing heavy metals, the method comprises the further step of subjecting the water removed during dewatering to electrolysis to produce metal-depleted water, and oxidizing the organics in the metal-depleted water to produce substantially pure water.
According to another aspect of the method according to the present invention, it is possible to produce a product sufficiently free of heavy metals to be safely useful as a fertilizer from sewage sludge which originally contained significant amounts of heavy metals. Also, according to the present invention it is possible to treat sludge or slime that is incapable of being dewatered by ambient evaporation so that it is capable of being dewatered by ambient evaporation, and by applying about 200 kwh or less of energy to the sludge or slime while removing water therefrom.
It is the primary object of the present invention to provide an apparatus and method that can successfully dewater a wide variety of slimes and sludges with a minimum energy input. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.